It is well known to cut tubing utilizing a cutting head comprised of fixed and movable dies, each having an aperture for receiving the outside surface of the tubing, and fixed and movable punches inserted in the tubing and concentrically located within the fixed and movable dies. The fixed and movable punches are located such that their line of separation lies within a shearing plane defined by the line of separation between the fixed and movable dies. The tubing is cut by moving the movable die and punch with respect to the fixed die and punch thereby causing a plastic deformation of the tubing material within the shear plane such that the tubing being held by the movable die and punch breaks away or is sheared from the tubing being held within the fixed die and punch. The movable punch experiences motion relative to the fixed punch as the tubing material surrounding the movable punch is moved by the movable die.
The motion of the movable die relative to the fixed die may be achieved by the successive operation of actuators connected to move the movable die in perpendicular directions parallel to the shear plane. The cutting head described above is known as a BREHM cutting head and is commercially available from Vulcan Tool Corporation, the assignee of the present application.
In order to obtain the optimum tube cutting characteristics, it is necessary that the line separating the fixed and movable punches inside the tubing be set and maintained in a predetermined alignment with the line separating the fixed and movable dies which defines the shear plane. There are several techniques for holding the punches in position. The punches may be mounted on a fixed arbor, a movable arbor, or may be held in place by a separate restraining device held in the tubing itself. The latter two techniques have practical limitations in terms of their application, and the present invention is directed to tube cutting machines utilizing a fixed arbor for holding the punches. For larger tubing diameters, the movable and fixed punches are held together by means of a tensioning rod which permits relative motion between the punches in a direction perpendicular to a central longitudinal axis common to both punches.
With a typical BREHM type tube cutting machine, tubing segments of standard mill lengths of up to 20 feet (6 m) are held on a separate loader mechanism having an independent power source. The loader mechanism directs the tubing through a cutting head on the tube cutting machine and onto a leading arbor contiguous with the fixed punch, that is, an arbor leading into an input side of the cutting head. The arbor length is sufficient to accommodate the tubing segment, e.g. 20 feet (6 m), and the arbor is anchored at an end of the machine opposite the cutting head. At its anchor point, the arbor contains an adjusting mechanism that permits the arbor and connected punches to be adjusted relative to the dies so that the parting line between the punches lies in the shear plane. A feed mechanism on the tube cutting machine feeds the tube segment back through the cutting head against an adjustable finger stop which measures a desired cut tubing length and signals the machine to cease the feed motion. Thereafter, the cutting head shears the desired length of tubing from the tubing segment.
Since the arbor supporting the tubing is over 20 feet (6 m) long, to minimize arbor droop, the arbor has a diameter that is substantially equal to the inside diameter of the tubing segment which makes loading and feeding of tubing more difficult. In addition, tubing segments with a camber or curve over their length either have to be straightened or cut into shorter lengths so that they do not bind on the arbor. Further, for a machine designed to handle tubing segments of 20 feet (6 m), the total length of the loader mechanism and the tube cutting machine is well over 40 feet (12 m).
To minimize friction in feeding the tubing segment over the punches, the fixed punch has a slightly larger diameter than the movable punch. However, during the loading operation, when the tubing segment is transferred over the movable punch and then over the fixed punch, the lesser diameter of the movable punch is next to the larger diameter of the fixed punch which forms an aggressive step which may score the inside surface of the tubing.
There are several aspects of BREHM type tube cutting machines which can be improved. The environment in which the tube cutting machine is used may have substantial temperature variations over its period of operation resulting in a longitudinal thermal expansion of the arbor which adversely affects the position of the punches relative to the dies. The longer the arbor, the greater the linear expansion from its point of support.
The tubing segments experience a bidirectional flow which requires that the tubing pass through the dies and over the punches twice. Not only does the tubing have two opportunities to be scored, but edges of the dies and punches are exposed to additional wear.
After being loaded onto the leading arbor, the tubing segment is cut into tubing pieces; and the tubing pieces are discharged from the same end of the machine that the tubing segment was loaded. The discharge point is the approximate midpoint of the combination of the loader mechanism and the tube cutting machine. Typically, in a high production environment, the loader mechanisms and tube cutting machines have only a walk aisle between them; and the discharged tubing pieces are manually carried to a material flow path at an end of the machine.